There are 10 US Post Offices in Clark County serving a total of 35,496 residents equating to 3,550 residents per post office. It's estimated that approximately 56,656 packages are handled across the 10 county post offices annually. For British letters to voters in Clark County in the 2004 Presidential election, see The Guardian. Clark County is a county located in the U.S. state of Ohio. As of the 2010 census, the population was 138,333. Its county seat is Springfield. The county was created on March 1, 1818, and was named for General George Rogers Clark, a hero of the American Revolution. Clark County comprises the Springfield, OH Metropolitan Statistical Area, which is also included in the Dayton-Springfield-Sidney-OH Combined Statistical Area."